Abstract : A correlation diagram shows that singlet D sulfur atoms may add to ethylene in a least-motion symmetry allowed process. The computed potential surface for the interaction of a sulfur atom with an ethylene reveals another local minimum in addition to the one leading to thiirane. This second minimum leads to the insertion product, vinyl thiol. The authors examine the potential surface in some detail to decide if both reactions branch from the same transition state or whether they are competing concerted processes. Similar potential surfaces were calculated for a S(triplet P) configuration. A minimum leading to addition was found, but the one leading to insertion is lacking. A ring-opened thiirane intermediate was computed. (Modified author abstract)